My sites were down or lagging at least 6 times within the past three weeks, with downtimes ranging from 1 to 3 hours per observed incident. A week ago I was moved to a new server after my fifth support call, and my insistence that no, I did not want to wait another 24 hours this time to see if there were more problems. The tech who responded to that last ticket said that the server in question (xevious) had been rebooted over 60 times in the prior 30-day period. With every ticket I reported I was told that they would begin an investigation.
Since being moved to the new server, I've had no stability problems at all.
So I'd say it's really luck of the draw. When Dreamhost works, it works very well and it's a great value. But if you end up on a bad server, you'll have repeated problems and you'll have to complain a lot just to make them aware that a problem exists.